Our previous work and that of others has shown that the smooth muscle relaxant, D-600, acts by preventing the inward movement of calcium accompanying the depolarization of the muscle cell membrane. Thus, D-600, may be a useful tool to determine the relative contributions of extracellular and intracellular calcium to tension development in smooth muscle. We intend to explore this possibility by comparing the effect of D-600 on contractions of the myometrium induced by isotonic potassium, oxytocin, acetylcholine and prostaglandins. Since the myometrium has relatively little capacity to store intracellular calcium as does, for exampl, vascular muscle, we shall also compare the actions of D-600 on drug induced contractions of the rat uterus and of rat arterial smooth muscle. We shall measure the magnitudes of the isometric tension, membrane potentials and calcium movements.